1. Field of the Invention
The invention relates to a photolithographic apparatus for imaging a mask pattern on a substrate and more particularly to a photolithographic apparatus that includes, a radiation source unit, a first optical system, an optical waveguide, and a subsequent second optical system and a mask table.
2. Discussion of Related Art
A photolithographic apparatus of the type described in the opening paragraph is known, for example from European Patent Application EP 0 658 810. Such an apparatus is used for manufacturing integrated circuits (ICs). In this apparatus, a mask provided with a pattern is illuminated and repetitively imaged on a photosensitive layer provided on a semiconductor substrate, for example, a silicon substrate. A large number of integrated circuits (ICs) must be formed on the substrate. After an image of the mask has been formed on the substrate, the substrate is to this end moved with respect to the mask through a distance which is slightly larger than the length or the width of the ICs to be formed, whereafter a subsequent mask image is made. This process is repeated until the desired number of ICs is formed. The apparatus described above is of the stepper type.
In current photolithographic apparatuses, it is desirable that the illumination beam has a maximal intensity so that the illumination period for each IC is as small as possible and the time of passage of the substrate through the apparatus, in other words the time required to illuminate all ICs, is as short as possible. Moreover, the aim is to image on larger fields and to decrease the dimensions of the smallest details to be imaged. Imaging on larger fields may be achieved, for example, by enlarging the projection lens. However, this renders the projection lens very expensive. Another way of imaging on larger fields is to make use of a photolithographic apparatus of the scanner type. In this type of apparatus, the projection lens does not need to be modified. An additional advantage of a scanner is the uniform quality of the image of the smallest details because aberrations caused by the lens are averaged during scanning. A result is that the output of components with structures having critical dimensions is higher.
In a scanner, a field to be illuminated is divided into imaginary sub-fields. Due to a coupled, continuous movement of the mask and the substrate, each time a different part of the mask is projected on the substrate so that the complete field will be illuminated gradually. When a complete field has been illuminated, the substrate is moved in this case through a distance which is larger than the length or the width of the complete field, so that the next field is reached. An example of a photolithographic apparatus of the scanner type is described in the article: "The future and potential of optical scanning systems" by D. A. Markle in Solid State Technology, September 1984, pp. 159-166.
The illumination time per field is longer for a scanner than for a stepper if the field size is the same in both cases. In fact, in a stepper, the illumination time per field is only determined by the available energy. In a scanner, the overscan time is also to be taken into account. Here, the scan time for illuminating the total field is not only determined by the field size but also by the slit width. Namely, there is a given time interval at the start and the end of the field to be illuminated, in which a part of the slit-shaped illumination field is not present above the field to be illuminated. The illumination time and consequently the time of passage of the substrate are thus dependent on the slit width of the illumination beam. However, systems in which a lamp is used as a light source have a relatively low optical efficiency. This is caused by the difference in throughput between the lamp and the slit geometry. In fact, the lamp illuminates light at all angles, while the slit geometry is anamorphous so that the geometry of the radiation field of the lamp deviates to a relatively large extent from the slit geometry.
If the fields are small enough to be illuminated completely with one pulse, a stepper is preferred as far as speed is concerned.